Microemulsions are described as clear, thermodynamically stable solutions that generally contain H2O, a surfactant, and an oil. H2O and CO2 microemulsions first appeared in the literature during the 1990's and appear to document the use of a surfactant to create these mixtures. The first reported microemulsions in highly compressible fluids (ethane and propane) under supercritical conditions was by Gale et al. (1987). This early work utilized the surfactant sodium bis(2-ethylhexyl) sulfosuccinate to emulsify the mixture of supercritical fluids and H2O. Microemulsions with supercritical CO2 were initially reported by Johnston et al. (1996) where the chemical ammonium carboxylate perfluoropolyether was used as the surfactant. Creating microemulsions from liquid CO2 (supercritical conditions) was originally accomplished by Eastoe et al., but required a partially fluorinated, di-chain surfactant bis(1H,1H,5H-octafluoro-n-pentyl) sodium sulfosuccinate (di-HCF4) and the stability of the mixture temperature and pressure dependent.
Although recent estimates (Milkov et al. 2003) put the global accumulations of natural gas hydrate at 3,000 to 5,000 trillion cubic meters (TCM), compared against 440 TCM estimated (Collett, 2004) for conventional natural gas accumulations, how much gas could be produced from these natural gas hydrate deposits remains speculative. What is needed to convert these gas-hydrate accumulations to recoverable reserves are technological innovations sparked through sustained scientific research and development. As with the unconventional energy resources, the challenge is to first understand the resource, it's coupled thermodynamic and transport properties, and then address production challenges. Carbon dioxide sequestration coupled with hydrocarbon resource recovery is often economically attractive. Use of CO2 for enhanced recovery of oil, conventional natural gas, and coal bed methane are in various stages of common practice.
Exchanging CO2 with CH4 has demonstrated CO2 to be preferentially clathrated over CH4 in the hydrate phase. During the exchange process, it has been observed that the mole fraction of CO2 in the hydrate phase was greater than the gas phase. This effect was quantified by noting the gas phase mole fractions of hydrate formers (i.e. CH4 and CO2) above 40% CO2 yielded hydrate phase mole fractions of CO2 in the hydrate phase greater than 90%. Pure CH4 and CO2 form hydrates, and their mixture also form hydrates. In forming mixed CH4 and CO2 hydrates, the CH4 molecules occupy both the large and small cages of these hydrates, whereas the CO2 molecules only occupy the large cages. Without hydrate disassociation, there is an upper limit to the substitution of CO2 for CH4 in hydrates. It has been estimated approximately 64% of CH4 could be released via exchange with CO2. In addition to equilibrium considerations, the heat of CO2 hydrate formation is greater than the heat of dissociation of CH4 hydrate, which is favorable for the natural exchange of CO2 with CH4 hydrate, because the exchange process is exothermic. There are considerable numbers of open literature publications on the CO2—CH4 gas exchange concept.